    The following abbreviations are defined as follows:    AWGN additive white Gaussian noise    BS base station    CINR carrier-to-interference-and-noise ratio    CP cyclic prefix    DFT Discrete Fourier Transfom    DL downlink    DLFP DL frame prefix    FDD frequency division duplex    FFT fast fourier transform    FUSC full usage of sub-carriers    MAC multiply-and-accumulate    MIMO multiple input multiple output    MISO multiple input single output    ML maximum likelihood    MS Mobile Station    OFDM orthogonal frequency division multiplex    OFDMA orthogonal frequency division multiplex with multiple access    PHY physical layer    RF Radio Frequency    RTG receive/transmit transition gap    SISO single input single output    SIMO single input multiple output    SNR signal-to-noise ratio    STC space-time coder    TDD time division duplex    TDMA time division multiple access    TTG transmit/receive transition gap    UL uplink    WiMAX Worldwide Interoperability of Microwave Access
IEEE 802.16 promises to provide fixed and mobile wireless broadband services with peak data-rates of up to 70 mbps. Release E of the 802.16 standards defines necessary enhancements to support full mobility. The air interface of the mobile IEEE 802.16e is based on OFDM modulation technology. The OFDMA mode of 802.16e is expected to be most widely deployed for the mobile broadband services. The OFDMA mode provides bandwidth scalability from 1.25 MHz to 20 MHz (or 1.75 MHz up to 28 MHz) and, with the help of different FFT sizes (128, 512, 1024 and 2058), it can provide wireless services with different throughputs and QoS.
OFDM modulation has been used in various wireless access technology, such as Wi-Fi (802.11a) and digital audio and video broadcast (e.g. DVB-H). However, full mobility and QoS support for multiple users in a typical point-to-multipoint (cellular) environment have mandated many changes in the physical layer (PHY) and the medium access layer (MAC) design. One addition relates to the use of various downlink (DL) preamble sequences, which allow the MS to uniquely identify a BS. There are 114 preamble sequences for each FFT-size which can uniquely identify one BS (sector). In the TDD mode of 802.16e the preamble is transmitted in the first symbol in the DL sub-frame. As in the 802.11a system, the preamble is also useful in achieving the system time and carrier frequency synchronization. However, the structure of the preamble is different in 802.16e and 802.11a.
In typical TDD-based OFDM systems, such as 802.11a or the 802.16d OFDM mode, the preamble is repetitive in the time domain. This property can be utilized to achieve low complexity synchronization based on delay correlation techniques. In FDD mode, the continuous transmission of OFDM symbols along with cyclic prefix can be used to achieve the symbol time synchronization.
In 802.16e OFDMA mode, the preamble contains data at every third sub-carrier. Since 3 does not divide the FFT size (power of 2), the preamble in 802.16e OFDMA mode does not repeat in time; although every third part of preamble symbol does exhibit good correlation. Also, during TDD mode reception, the UL sub-frame follows the DL sub-frame. DL and UL sub-frame are separated by TTG and RTG gaps, which are not integer multiple of an OFDM symbol. This makes the conventional CP-based symbol time acquisition very challenging. Also, since the MS can receive transmissions from multiple BSs, the delay correlation based estimation of frame time (based on high correlation between parts of the preamble symbol) does not provide a good estimate of frame time. The delay correlation search typically returns a large window of possible frame start events.
Cross-correlation with the known preamble sequence, on the other hand, provides more accurate timing information. However, since there are a large number of possible preamble sequences, an exhaustive cross-correlation search would have prohibitively high hardware complexity. Time domain cross-correlation processing also requires that the preambles are either stored in the time domain (after IFFT), leading to higher memory requirements, or generated on the fly, leading to an additional IFFT for each preamble search.
Moreover, since the preamble sequences are randomly searched, they cannot be generated on the fly and must be stored. It is beneficial to store the preamble data in the frequency domain, as the preamble data modulation in the frequency domain is BPSK (1-bit). Several conventional OFDM synchronization techniques include the following.
A preamble-based packet synchronization scheme has been widely used in 802.11a systems. Two different types of preamble sequences are transmitted in 802.11a (short and long). The short preamble repeats about eight times (four times in one symbol and then transmitted twice), while the long preamble symbol repeats twice. The preambles are unique for all the access points in 802.11a. Typically the repetition property of the short preamble is exploited to obtain coarse time synchronization via delay correlation. The fine time synchronization can be achieved by searching for long preamble symbol. This can be obtained via delay correlation or cross correlation. The carrier frequency offset is measured by measuring the sample phase difference between repetitive parts of short preamble.
In terrestrial broadcast systems such as DVB-H, a CP-based delay correlation is implemented to obtain the symbol time. The carrier frequency offset can be measured in two steps: during a fractional frequency offset measurement using the phase of the CP correlation, and during an integer frequency offset measurement using the pilot rotation in the frequency domain.
However, in the TDD OFDMA mode, the above techniques either cannot be applied or return inaccurate time estimation.
Prior to this invention, no truly satisfactory procedure has been proposed to achieve DL synchronization in a device deployed in an IEEE 802.16e (WiMAX) communication system.